This invention relates generally to the field of adaptive optics and, more particularly, to techniques for sensing aberrations in optical wavefronts. The term adaptive optics refers to optical systems in which one or more elements are modified to adapt to changing conditions, such as continuously changing atmospheric conditions that cause aberrations in optical wavefronts. In an adaptive optical system, aberrations are sensed in a wavefront sensor, and error signals are generated for the control of a corrective device, such as a deformable mirror, which compensates for the detected wavefront aberrations and thereby continuously adapts the optical system to the changing conditions. In a number of applications of adaptive optics, it is necessary to sense the aberrations in a very weak optical beam. These applications include transatmospheric imaging in astronomy, optical communications links, high-power optical beam propagation, and holography.
In these and other applications, the wavefront of an incoming optical beam is sensed in order that in incoming or outgoing beam can be corrected for optical aberrations. An important class of wavefront sensors utilize point diffraction interferometry to obtain phase front information from the incoming wave. In this approach, a portion of the incoming beam is split off and passed through a spatial filter to obtain a clean, uniphase reference beam that can be interfered with the remaining portion of the aberrated incoming beam, to obtain the desired phase front information. This technique makes use of the principle that a pinhole spatial filter produces a practically perfect spherical wavefront. The difficulty with the approach is that, for many applications, there is insufficient optical power in the reference beam after splitting off from the incoming beam and passing through the pinhole spatial filter.
It has been proposed to use conventional laser amplifiers to amplify a portion of an input or probe beam. It has also been suggested that some kind of amplification be employed to operate on a spatially small portion of an input beam, by first spatially filtering, then amplifying the beam. However, neither of these solutions has provided a satisfactory solution to the problem.
It will be appreciated from the foregoing that what is needed is a technique for generating a relatively strong, coherent, uniphase reference wave for interfering with an aberrated beam. The present invention satisfies this need, as will become apparent from the following summary of the inventive principles.